I disagree, you said you plan to add another 660ti in the future. If this is the case I would recommend the Corsair AX 860i. First off, I agree with you for looking at seagate or corsair when developing your choices, however, for SLI a 650w is not enough. Use this PSU calculator from ASUS "http://support.asus.com/powersupply.aspx" it will help you determine how much wattage you need. Dont go too low on your power supply. All your components need to be utilizing as much power as they request based on the load on the system. If you go too low you risk performance losses or not even booting. I wouldnt go lower than 850w for a PSU and if you can pick a particular 850w PSU then the Corsair AX 860i is the best one out. Additionally, the Corsair AX 860i is fully modular so you can do extremely nice cable management in your system. I'd imagine that if you are planning on two 660ti's you probably want to show them off, which means you need good cable management.
Side note: I have the Corsair AX 1200i so I am speaking from experience with the models (obviously at a higher wattage). The AXi series PSU's are platinum efficiency, which is the best on the market.
Here's some pics as proof to show you my PSU.